1. Field of the Invention
The present invention relates to a zoom lens focusing apparatus in a camera or the like.
2. Description of Related Art
In a zoom lens in which focusing is accomplished by moving a movable lens in a master lens section, a focal position of the master lens section moves when the magnification is changed in a zoom lens section even if a subject distance is constant. Therefore, in order to maintain this focal position constant, it is necessary to calculate the focal length so as to automatically correct the position of the movable lens of the master lens section. To this end, for example, as disclosed in Japanese Patent Unexamined Publication No. 57-186872, Japanese Patent Unexamined Publication No. 52-114321, and so on, there is provided a moving-range calculating circuit in which positional information of the movable lens of the master lens section depending on the focal length as well as the subject distance is incorporated therein in the form of a calculation formula or a table so that the focal length is calculated to thereby correct the position of the movable lens. Generally, a digital circuit is used to hold the positional information in the form of a calculation formula or a table.
When the digital circuit is used, however, the precision of control is influenced by the resolution of an analog-to-digital converting circuit and a digital-to-analog converting circuit. In that case, if the resolution is made high the converting circuits become complicated, while if the resolution is made low on the contrary it becomes impossible to perform continuous control so that the picture becomes poor. In an automatic focus control mode, generally, it seldom occurs that the picture becomes poor since information concerning the focal length and the aperture is put in a servo loop so as to achieve focusing as a whole. In manual focusing, on the contrary, since no correction means is provided, there is a marked tendency that the picture becomes poor. Moreover, the converting circuits are low in stability and can not be simplified.